Unit 2Elements of Chemical Change

2-11. WATER

Water is the most important liquid to all living organisms.
It comprises about 57 percent of your body weight. It is the solvent for or is contained
in most of the nutrients your body requires for growth or maintenance. It is also the
primary vehicle for almost all liquid pharmaceutical preparations. Because of the inherent
importance of water in the practice of medicine, it is essential to acquire some knowledge
of the properties of water and aqueous (water-based) solutions.

Properties of Water. We are all familiar with some properties of water. We know that generally water is a
bland-tasting, colorless liquid. Other specific properties of water are of importance in
medicine.

Its boiling point is 100º C (212º F).

Its freezing point is 0º C (32º F).

It is a polar solvent (dissolves ionic compounds).

Generally, it is chemically inert (unreactive) in biological
or drug systems.

Importance of Properties. The
properties above are the specific reasons that water is so valuable to living systems and
to pharmaceutical preparations. The wide difference between the freezing point (water as
ice) and the boiling point (water as steam or vapor) indicates that water will be a liquid
at most of the temperatures encountered under normal conditions. An example should help
emphasize the importance of these properties. If we wanted to prepare a liquid drug
solution for a patient who could not swallow capsules, we used a liquid vehicle with a
freezing point of 25° C (77° F) and a boiling point of 30° C (86° F), we would be
giving the patient a worthless product. As the patient left home, the drug solution would
boil if it were a normal summer day (temperature = 86° F), and when the patient entered
his air-conditioned home, the remaining solution might become a solid which could not be
poured from the bottle. We also want our vehicle to be as unreactive as possible so that
only the drug is exerting a pharmacological effect.

Structure of Water. The
properties of water may best be explained by examining the structure of the water
molecule. The water molecule consists of two hydrogen atoms bonded covalently to one
oxygen atom. The three atoms are bound together as shown below.

This arrangement leads to an electron-rich atom, oxygen, on
one end and two electron-poor atoms, hydrogen, on the other end. This results in a
molecule that resembles a bar magnet in that it has a negative pole and a positive pole,
as shown below.

Actually, there are not distinct electrical charges on the
molecule, only partial charges, referred to as d+ and d- (the Greek letter delta, d, meaning partial). While these
charges are only partial, they are still strong enough for water to be referred to as a
polar molecule, meaning that it has a positive and negative end.

Hydrogen Bond. The polarity of
the water molecule gives rise to an unused type of bond between water molecules, the
hydrogen bond. This bond is the electrical attraction between the partially negative
oxygen atom of one molecule and the partially positive hydrogen atom of another molecule.

The hydrogen bond is a very weak attraction about 1/10 to
1/20 the strength of the hydrogen-oxygen covalent bond. The hydrogen bond explains why
water has such a high boiling point in relation to other compounds of similar molecular
weight. For example, methane (CH4, molecular weight = 16) boils at a
temperature below 0ºC, while water (molecular weight = 18) boils at 100ºC. Methane does
not exhibit hydrogen bonding.

Water Purification. We are all
familiar with some of the ecological problems facing the world today. Water is subject to
mineral and biological contamination. Since we will often be using water in the
preparation of our products, we must be concerned with its purity and the methods utilized
for its purification. There are two common methods of water purification used at Army
medical treatment facilities--distillation and ion exchange.

Distillation. Distillation is the process of boiling water,
collecting the vapor, and then condensing the vapor back into water. Minerals and some of
the bacterial contamination will remain in the boiling vessel as a residue. Very pure
water may be prepared by repeating the distillation process several times. If sterile
water is desired, the water must be sterilized, because the process of distillation does
not necessarily sterilize water.

Ion exchange (deionization). Less common than distillation
because it is less efficient, ion exchange involves passing water through a column
containing a charged resin. Ions in the water are held by electrical attraction and are
thus removed from the water.